Process of separating air and similar gaseous mixtures by liquefaction and rectification



July 23, 1935.

"WIEN-rola 507;," 6a rhode?" ATTORNEYS Patented July a3,` 1935 :UNITED AvSTATES PROCESS oF 4surliml'rllvo am AND -slMI- LAR GAsEoUsMlx'ruaEs Br LIQUEFAC- 'rroN AND lusc'rrrrcn'rrr)NY Eugne Gomonet, Paris, France, assignor to Airl Reduction Company, Incorporated, New York,

N. Y., a corporation' of New York Application April 30, 1934, Serial No. 723,030 A In France May 4, 1933 sclaims.' (cica-1755iA This` invention relates to the 'separation by liquefaction andl rectification of gaseous mixtures, comprising at least three constituents one of which occurs ina quantity substantially less` 5 than each of the first two, and has a condensing temperature located between those' of the 4first two. The process,thereforevapplies vparticularly to atmospheric air in which argon occurs-in -a substantially smaller quantity than either oxygen or nitrogen, and its condensing temperaturev is located between these last' two gases. For greater simplicity, we Vshall refer in the following treatise only to atmospheric air. But, it is understood that the process which will` be described can be applied in a general way to `gaseous mixtures of similarcomposition, such as were defined above.

It is common practice to`sepa1'jate air into oxygen and nitrogen by introducing it in the gaseous state, after having been cooled, into an intermediate level of a rectifier the lower portion of which is heated' and in the upper portion of which liquid nitrogen is admitted. But in the carrying out of this process for separating air, the quantity lof airthat can be separated into oxygen substantially free from nitrogen.v and nitrogen sub- 4stantially free from oxygen is smaller than that which we should be able to separate in view of the composition of the liquid mixture of oxygen v and nitrogen, whichicontains very little argon, in

equilibrium withlgaseous atmospheric air. The reason is that argon accumulates in the column accordingto the process'polnted out in U. S. Patn ent No. 1,068,219 of Claude. Owing to this accumulation of argon, 'the liquidand the gas in the column are, at the level at which the air is introduced, not binary mixtures of oxyge'nand nitrogen, but ternary vmixtures of oxygen, nitro-fx gen and avery substantial proportion ot argon.

40 For such ternary mixtures' the ratio of the proportion of oxygencontained in the liquid'to the proportion of oxygen contained in tle gas is not sogreat-as it would be iitherey 4were no argon. It has been suggested-in Ithe abovev noted U. S.

, 405 Patent No. 1,068,219, 'to provide a special outlet for argon at a-level immediately above one of the ,trays of the lower part of4 the rectifier column for withdrawing some gaseous loxygen rich inar'gon.

By applying-.the process of. this U. S. patent tothe process of separating the air into its constitu= ents by the introduction of air at an intermediate level of the rectifier, we should effectively prevent the presence oi substantial quantities of argon in the column at the place of the introduction of thetreated air, and we could therefore introduce into the rectifier an excess quantity of air substantially as great as if the treated air were devoid of argon. On the other hand, a fair propor- 'tion of4 the oxygen contained inthe treated air would be'recovered not as substantially pure oxy- 5 gen, but as a mixture of oxygen, argon and a certain proportion of nitrogen which would have to be subjected to a new treatment`for example l the one described inthe U. S. Patent No. 1,638,005

of LeRouge if we Wish to obtain the oxygen in 10' substantially pure state. Consequently in this case also, the liquid .nitrogen admitted at the top of the rectifier would not enable us to separate into oxygen' substantially free from nitrogen and into.- nitrogen substantially free from oxygen that quantity of air which iin lView of the ,f

composition of that liquid mixture of'oxygen.

and nitrogen, containing very little argon, which is in phase equilibrium with gaseous atmospheric air) should be rectified by. the reflux liquid nitro@y 20 gen.

To sum up,'wl1`ether one doesY not eliminate the -argon from the rectifier column Yby a special'out-l let, or whether one eliminates it by the process Y of the U. S. Patent No. 1,068,219, the presence of 25 argon in the treated air has at all events the resultof increasing Kthe amouniof liquid nitro- A gen necessary to obtain a definite quantity of 'oxygen substantlallyfree from nitrogen. This liquid nitrogen is obtained by compressing nitro- 30 gen to suicient'pressure'to permit liqueiaction in a'bath of a liquid less volatile than itself, generally the liquid oxygen from the bottom of the rectifiercolumn. The existence of argon in the air therefore entails the disadvantage in-thewell -35 known above mentioned processes, ofincreaslngl the energyn'ecessary for the separation of the treated air. l l A It is'the obiect`of the invention to avoid the f dliculties mentionedv and to provide a simple and 40 effective process for separating the' gaseous mix? ture into oxygen and nitrogenxpractically pure. According to the present invention, the process 'consists in introducing into the column a quan-- tity of `air in excess'of the quantity which, `(in 45 'view ot tlerespectivecompositions of the air and the liquid mixture of oxygenand nitrogen inA .phase equilibrium' therewith) might be rectified `by the quantity of liquid ,nitrogen admitted'at the topofthe column and in withdrawing from 50 the column at the level of, or a level somewhat above the place where the air isv introduced, a

quantity of gas richerin argon than the air land, in amount.l substantially equal to that excess air, A

The respective argon contents of the Aliquid 55 and of the gas of the column both decrease in proportion as we move upwardly from the level of air introduction. As a result, in order to lower to a determined value the argon content of the vapor of the column at the level where the air is introduced, we must withdraw all .the more gas and introduce a corresponding` excess of air so much the greater as the point of withdrawal is more remote from the place where the air is introduced. But the introduction and withdrawal of air in excess require a certain expenditure of energy, even if the column is at atmospheric pressure, and furtherfore the recovery of the withdrawn cold is necessarily imperfect; For these various reasons it will be advantageous to withdraw the gas enriched in argon at a place which will be as near as possible the place where the air is introduced and will only be separated therefrom, for example, by one or two rectifier trays.

The gas enriched in argon can be withdrawn even at the level of the introduction of the treated air, that is, between those two rectifier trays between which the entrance for air introduction is 1ocated. In this latter case, the inlet orifice and the outlet orifice naturally must be far removed' from one another but so ass/till to permit good.,

' `Vuse of an excess of air costs somewhat in cold and energy, its quantity7 can remain .in practice, for example, between one-third and one-fifth of the sum total of the air treated by the apparatus. 1

The process according to the present invention is particularly applicable when therectiiler column is the low pressure column functioning at atmospheric pressure of a rectifier apparatus with both high and low pressure.. In this two` fold rectification the quantity of liquid. nitrogen at ones command for the low pressure rectication is theoretically overabundant so that it seems that one ought to be able to introduce 'a considerable quantity of air into the low pressure column, without ceasing to obtain practically pure oxygen and nitrogen. But in practice the quantity of air which one thus can blow in is far lower than the theoretically calculated quantity when we `consider the air as a binary mixture o f oxygen and nitrogen. This is attributable to the presence of argon in the column which tends to cause the oxygen contents of the liquid and oi the vapor in the column to approach each other as has been explained before'. If we introduce in accordance with the present invention a quantity of air great-A er than that the rectification of whichis theoretically possible and withdraw gas at'thelevel or very nearly the level of introduction, tor example, above the -tray situated right above this orifice, the dinerence between the quantity of --vantageously introduce without withdrawal.

The liquid which is in equilibrium with the air introduced into the high pressure column whichwe deliver to'the'low pressure column the exchanger D Athe liquid nitrogen drawn up at this low pressure in equilibrium with gas poorer in oxygen than air so that the introduction oi air into the'low pressure column must take place slightly below the place where the liquid rich in oxygen is admitted. To withdraw a gas having an oxygen content as close as possible to that of air, one must eiect the withdrawal of the gas from the' low pressure column preferably between the place of air introduction and the place where the liquid rich in oxygen is delivered.

In the accompanying drawing, the gure shows schematically. the application of the process in the case of rectiilcation of the air with high and low pressure.

. In this figure. A denotes the column under pressure and B the column at atmospheric pressure. 2.72 rn3 of compressed and cold air under a pressure of between 4 and 5 atmospheres absolute is admitted through the p ipel at the lower portion of the column A. At the bottom ofy column A one collects a liquid containing about 40% oxygen, the quantity of which corresponds to 1.43 in:i of gas and which is conducted through pipe 2 and expansion -cock l2 into column B. At the top of column Aone collects a quantity of liquid nitrogen which corresponds to 1.29 m3 of gas, and conducts it through pipe 3 and expansion cock 3 to the top of column B. Through pipe 4 one blows into column B at .a-point situated-three. o'r four rectification trays below the place where the liquid rich in oxygen is admitted.

3.63 m1 of cold air at atmospheric pressure.

Through pipe 5 one withdraws right above the irst rectification tray situated above the inlet for blowing in air-1.59 m3 of a gas havingrpractically the same content as the air in oxygen and nitrogen, but richer thanair in argon. Atthe lower part and at the upper part of column'B, one withdraws respectively 1 m3 of pure gaseous .Oxygen by pipe 6 and 3.7.6/ m3 of pure gaseous nitrogen throughpipe 1J 'I'he gaseous oxygen is used for sub-cooling in exchanger C the liquid rich in oxygen drawn up through/.the pipe l. Likewise, the gaseous nitrogen is used to cool in through the pipe l land the liquid rich in oxygen which was previously cooled in the exchanger C,

The process according tothe presentinvention is independent oi the processes used for the recovery of the cold of the gases leaving the recti- 'iler column or for the creation of the cold intended to compensate the losses of cold.l In casev it is desired to obtain very pure oxygen and nitrogen. one will advantageously employ temperature exchangers, preferably to regenerators with alternating action, for the recovery of thev cold. To facilitate the drying of the treated. air and the exchange of temperature between vthe two gases circulating in the exchanger. it will then be advantageous to compress the air atl a pressureg higher than that which prevails in the column, and after passing intothe exchanger to expand at least part thereof with outside work to the pressure which prevails in the column for the l production of cold. v

,'Various changes may bemado in the procesa and apparatus as described .without departing `from the invention or sacrificing any ofthe adj vantages thereof.

LADrooessforobtalningsubstantlallypm voxygen and nitrogen from. atmosphericair by liquefaction and rectiiica'tion,v comprising withdrawing a gaseous mixture loi! oxygen and nitro-- rectification and introducing imseparated atmospheric air at substantially the same level and in quantity substantially greater than that of the gaseous mixture withdrawn.

2. A process for obtaining substantially pure oxygen and nitrogen from atmospheric air by liqueiaction and rectiiicatlon, comprising withdrawing a gaseous mixture rich in argon from an intermediate level ot rectication and replacing said gaseous mixture withdrawn with air, introduced at substantially the saine level, and in quantity substantially greater than that of the gaseous mixture withdrawn.

3. A process for separating the constituents of a ternary gaseous mixture. one constituent being presentin relatively small amount andl having a boiling point between those of the other constituents by liquefaction and rectiiication, comprising withdrawing a gaseous mixture containing a relatively large amount of the constituent having an intermediate boiling point at an intermediate level of rectiilcation and introducing at substantially the same level of rectification a mixture yof the gases containing a lrelatively smaller amount `of the vconstituent having the intermediate boiling point and `in quantity greater than that of the gaseous mixture withdrawn. 

